Applications of second harmonic generation (SHG)/sum-frequency generation (SFG) imaging for biophysical characterization of the plasma membrane

Abstract

The plasma membrane is a lipid bilayer of < 10 nm width that separates intra-and extra-cellular environments and serves as the site of cell-cell communication, as well as communication between cells and the extracellular environment. As such, biophysical phenomena at and around the plasma membrane play key roles in determining cellular physiology and pathophysiology. Thus, the selective visualization and characterization of the plasma membrane are crucial aspects of research in wide areas of biology and medicine. However, the specific characterization of the plasma membrane has been a challenge using conventional imaging techniques, which are unable to effectively distinguish between signals arising from the plasma membrane and those from intracellular lipid structures. In this regard, interface-specific second harmonic generation (SHG) and sum-frequency generation (SFG) imaging demonstrate great potential. When combined with exogenous SHG/SFG active dyes, SHG/SFG can specifically highlight the plasma membrane as the most prominent interface associated with cells. Furthermore, SHG/SFG imaging can be readily extended to multimodal multiphoton microscopy with simultaneous occurrence of other multiphoton phenomena, including multiphoton excitation and coherent Raman scattering, which shed light on the biophysical properties of the plasma membrane from different perspectives. Here, we review traditional and current applications, as well as the prospects of long-known but unexplored SHG/SFG imaging techniques in biophysics, with special focus on their use in the biophysical characterization of the plasma membrane.

Long-ranged heterogeneous structure in aqueous solutions of the deep eutectic solvent choline and geranate at the liquid–vapor interface

“…More interesting, perhaps, the density oscillation, aggregation, and structural observations could be examined with sum frequency generation, second harmonic generation, and other surface specific spectroscopies. 49,50 Furthermore, electron holography could be used to examine molecular structure, electrostatic potential, and other properties at sub-ångstro ¨m resolution with respect to the interface. 51,52 Such experimental studies of aqueous CAGE and other similar ionic liquids and deep eutectic solvents in concert with the computational protocols presented here promises important insight into the physical underpinnings of the interfacial behavior of these materials and serves as a simple model for their solvent properties.…”

Biophysical Reviews: Turning the page from 2022 to 2023

“…Exploiting the SHG principle in SHM involves the application of a specific wavelength filter to light observed from an illuminated sample. SHM has been a popular topic within the journal, with a number of reviews on related topics being published over the last 15 years (Cox 2011;Kato 2019;Mizuguchi and Nuriya 2020;Pallen et al 2021). The current work describes the history of the SHM field and its use in examining various fibrous structures such as collagen and microtubules (Aghigh et al 2023).…”

Electric-field induced second harmonic generation responses of push–pull polyenic dyes: experimental and theoretical characterizations

“…1 In particular, the exogenous labeling of lipid bilayers by amphiphilic potentialsensitive dyes displaying large second harmonic generation (SHG) responses is at the heart of high resolution imaging microscopy techniques. [2][3][4][5][6][7][8][9] SHG probes are usually designed by functionalizing the two extremities of a p-conjugated linker with electron-donating and electron-withdrawing substituents, which provide the asymmetry required for quadratic NLO phenomena. The elaboration of SHG chromophores based on this dipolar architecture, [10][11][12][13] including responsive systems such as NLO switches, [14][15][16] has been the object of intense research in the last 30 years.…”